Advanced Models for Practical Devices

Abstract

In this chapter, control of the band alignment in nonideal case is discussed. A parameter $\Delta$, potential gap at the interface is introduced to consider nonideality. Several models on the origin of nonideality are explained: metal-induced gap state model, disorder-induced gas state model, interface-induced gap states model. In principle, interface states are generated at the interface and electrons fill these states from the lowest states. The energy level of interface states that is composed of 50% of the valence and 50% of conduction bands, is called the charge neutrality level (CNL), because if electrons fill up to CNL, the semiconductor becomes charge-neutral. The highest energy level electrons fill is usually different from the Fermi level, which results in the potential gap $\Delta$ in any models. The origin of the interface states differs from model to model. Potential gap $\Delta$ is related to S parameter and S parameter is related to optical dielectric constant of semiconductors or insulator in contact with the metal. Experimental examples are demonstrated to show that S parameter is modified by inserting ultrathin insulating layer between metal and semiconductor. It confirms that the modification of band alignment using S parameter is effective.